In fast, motor-driven watercrafts that are equipped with a surface drive, the propeller shaft can pivot in all directions around a hinge point with the drive shaft from the motor or the transmission. The motor and transmission are located in the ship's hull. The depth of immersion of the propeller and with it the conversion of drive energy into thrust is changed by pivoting the propeller in a vertical plane parallel to the longitudinal axis of the watercraft. This pivoting of the propeller shaft in the vertical plane is called trimming, and the amount of pivoting is called the trim angle. At higher headway speeds and with the propeller only partially immersed, the surface drive reaches its best efficiency. The optimal trim angle thus depends on the headway speed of the watercraft and is approached manually in ordinary watercraft, with the corresponding inaccuracy. In addition, manual trimming burdens the boat's skipper in addition to his other tasks, which likewise makes it difficult to set the optimal trim angle.
The prior art describes an automatic trim control for a surface drive that automatically adjusts the trim angle as a function of the particular operating range. The operating ranges are defined by the position that the watercraft assumes in the water at different speeds.